In fiber-optic communications there is a need for optical switching of light signals from fiber to fiber for path provisioning (creating data routes) at Optical Network Nodes (ONNs). These connections are intersections of major pipelines between Network Access Stations near the user networks. Long haul use patterns are fairly regular hence connections at ONNs are relatively constant and persist for minutes to hours. The industry is developing a solution based on the free space optical cross connect switch (OCX) to facilitate the ONN switching function.
With the development of digital wavelength division multiplexing (DWDM), the number of channels needed for optical switching can become very large. For example, 6 fibers carrying 160 wavelengths each results in 960 switchable light paths; hence OCX arrays of 1000 by 1000 ports can be needed. The micro mirror actuator systems of prior art designs include movable mirror elements that are typically rotated through a small angle in one or two planes by pairs of electronic actuators. The actuator may be mounted on a surface underlying the mirror and directly below unattached portions of the mirror. When a current or voltage is delivered to the actuator the unattached edge of the mirror is drawn toward the actuator by an electrical force moving the unattached portion of the mirror towards the underlying surface. Such devices usually use active closed loop electronic drivers that are always in an “on” state in order to hold the unattached mirror edge in a particular position. These devices require continuous consumption of power, to maintain the position. With each actuator dissipating just a fraction of a watt, the system load can total hundreds of watts of power for large actuator counts. This power generates heat that must be cooled and the total power load must be backed up in case of disruption, with battery and generator systems causing additional complexity and cost.
Prior art MEMs constructions have multiple deficiencies. FIG. 1 depicts a typical MEMs construction in which a gimbaled mirror assembly 1000 includes a movable mirror 1002 suspended on fine gimbaled structures or thin hinges 1004 and 1006 for rotation about a Y axis and suspended on fine gimbaled structure or thin hinges 1008 and 1010 for rotation about a X axis. The MEMs gimbaled mirror 1000 is mounted to an actuator layer 1012, shown cut away, which includes actuator elements for attracting a free edge of the mirror toward the actuator layer 1012.
In FIG. 1, an actuating force applied to the mirror 1002 from the actuating surface 1012, near a point A, draws a free edge of the mirror toward the actuator surface 1012 in the direction of the arrow shown at A. The mirror 1002 pivots about the Y-axis at the gimbals 1004 and 1006 such that the mirror at point B is raised with respect to the actuator surface 1012 as shown by the arrow at B. In the other axis, an actuator on the actuator surface 1012 near a point C draws another free end of the mirror in the direction of the arrow shown at C. The mirror 1002 pivots about the X-axis at the gimbals 1008 and 1010 such that the mirror at point D is raised with respect to the actuator surface 1012 as shown by the arrow at D. Single axis devices are also known for providing tilt about a single axis only. The actuator devices may employ electro-static, electromagnetic piezo-electric and mechanical actuator forces.
The gimbaled mirror assembly 1000 may be formed of a silicon or poly-silicon structure deposited or otherwise formed onto the actuator layer. In order to provide a reflective surface on the mirror 1002, a Metal Oxide Chemical Vapor Deposition comprising a coating of e.g. aluminum, silver, gold or another reflective material coats the surface 1002. The reflectivity of such layers is usually limited to about 96 to 98%. One example of a prior art MEMs device like the one shown in FIG. 1A has been described by Lucent Technologies and may have a mirror diameter in the range of about 100–500 μm (0.025–0.127 in.).
FIG. 1A depicts a plurality of optical switching mirror assemblies 1000. One commercially available example provides a 64 by 64 MEMS optical switch module having an operating temperature range of 5 to 70 degrees Centigrade, listed mirror switching time of 20 ms, with a power dissipation consumption of 15 watts. Insertion loss is 6 dB max (e.g. 75% losses); optical return loss is 30 dB, and cross channel isolation is 50 dB. Optical power transmission is limited to 31 milliwatts per port. In the particular example of FIG. 1A, each device 1000 is centered with respect to rows 1014 and columns 1016. Such an arrangement provides a poor packing density for each device leaving a low mirror area to total area ratio. Optical switching system, arrays of as few as two mirrors up to as many as 1024, or more, separate mirror elements may be required to be operating in an optical network switching hub.
One problem with the device shown in FIGS. 1 & 1A is that the mirror is surrounded mainly by air and lacks any conductive path to remove heat. This is one reason that conventional MEMS mirror devices are limited to low power, e.g. only 31 mw in the above example. Since the reflective surface of each mirror is typically limited to about 96 to 98% reflectivity, 2 to 4% of the light energy reaching the mirror may be absorbed by the mirror substrate or scattered, thereby heating the mirror substrate and the surrounding elements.
MEMS mirrors are also thin and subject to surface distortion caused by thermal stress such as may result from the heat absorbed by the substrate and by the always-on actuators. Other surface distorting factors include mechanical forces developed during actuation and release of actuation and even sagging due to the MEMs mirror low stiffness. Vibration and shock loads may also lead to transient mirror surface distortion. Mirror surface distortions may cause beam distortions, e.g. wave front aberration, scattering and optical power fluctuations, possibly resulting in increasing optical losses, signal errors and channel cross talk. A mirror system of the highest attainable reflectivity, of the highest obtainable surface flatness or accuracy of surface figure and of the highest possible stiffness and with less optical energy absorption and or better heat dissipation capability would be beneficial and could be used to reflect much higher beam powers than are now reflected by MEMS systems.
Reflective coatings in excess of 99.5 percent are realizable using multi-layer optical coatings under good process conditions. Such coatings are typically coated onto optical surfaces such as glass and metal and would be advantageous on optical switching mirrors to reduce scatter and absorption. However, these coatings have heretofore not been applied to conventional MEMs or other micro mechanical mirrors because the mirror structure is either too delicate for the coating environments or the mirror material is not compatible with accepting such coatings. Higher reflectivity coatings could reduce absorption in the mirror substrate. Mirror surfaces with a flatness of 1/2–1/10 wave at the wavelength of the reflected light are routinely provided using conventional metal and glass mirror substrates by polishing. However, these polishing techniques have heretofore not been applied to conventional MEMS devices because the mirror structure is too small, too delicate, and not stiff enough or because the mirror material is not compatible with the polishing techniques.
Furthermore, to eliminate the actuator hold power required to hold a mirror stationary in prior art mirror actuators, a capability to latch or hold a mirror in a selected position without the need for electrical power would be most desirable in the optical switching field to further reduce power consumption and heat dissipation in the region of the mirror. Mirror actuating systems with a non-power consuming latch mode are not known for use with conventional mirror actuator devices. In large arrays in particular, it becomes important that the beam steering elements do not themselves place limits on the packing density of the physical parts so that a smaller overall unit size is achievable. Unit size and especially a high ratio of mirror surface area to total surface is important for closest packing arrangements in higher density arrays.
It is also desirable in optical switching systems to provide the lowest possible switch movement, settle and latch time for moving a mirror to a new position.
In other areas, the prior art teaches a diversity of beam steering or scanning devices used for single, dual and even three axes scanning of a radiation beam. FIG. 2 depicts a dual axis scanning device having first and second rotating mirrors 1018, 1020 mounted on first and second rotation elements usually comprising a limited rotation motors or galvanometers 1022, 1024. Each limited rotation motor 1022, 1024 is controlled by a servo or other style microprocessor controlled controller 1026 to rotate the mirrors through a desired angular range of just a few degrees up to 45 degrees or more. A radiation source 1028 and a source controller 1030 provide a radiation beam 1032 for directing onto a two-dimensional plane area 1034. A lens 1036 may be provided to focus the beam 1032 in the plane 1034. Each mirror 1018 and 1020 is individually controlled in its rotation angle to direct the radiation beam 1032 to any desired point in the plane 1034. In this example, rotation of the mirror 1018 scans the beam along the X-axis of the plane 1034 and rotation of the mirror 1020 scans the beam along the Y-axis of the plane 1043. Such a system as is depicted in FIG. 2 is capable of deflecting very high power optical beams without damage and provides very accurate beam placement capability. One drawback of the system is that it has heretofore been difficult to miniaturize.
Readers may find the following to provide further useful context for understanding the present invention. Yagi et al's U.S. Pat. No. 6,154,302 discloses a light deflection device in the form of a reflective or refractive surface supported on a hemisphere or half-ball member, which is supported by its hemispherical surface on a dielectric liquid layer within a conforming socket or cavity of a base member. The ball has positive and negative chargeable regions made of different materials, so that turning torque is applied to the ball by the electrostatic force from an electric field in the dielectric liquid created by electrodes distributed around the cavity in the base member, to which a suitable voltage is applied. The electrostatic turning torque alters the tilt or angular position of the hemispherical member axis relative to the base member, until a position of equilibrium corresponding to the applied voltage is reached. A variation contemplates a magnetic film on the ball, and an electromagnet on the base member, where rotation is controlled by magnetic force between the film and the electromagnet. Friction in some cases and lack of any turning torque in other cases is said to hold the ball stationary in the cavity after the electric field is extinguished. An array of such elements is also shown. One drawback of the invention by Yagi et al. is the need immerse the hemispherical elements within the dielectric liquid layer.
Sakata et al's U.S. Pat. No. 6,201,644 B1, of which Yagi is a co-inventor, describes a light deflection device and optical switching array, using a spherical body or half-ball and cavity similar in general appearance to Yagi's, with similar driving mechanisms for tilting the ball and with the same dielectric liquid layer.
Donelan's U.S. Pat. No. 4,436,260 illustrates a refracting optical scanner based on a hemispherical and a cylindrical shaped element with a mating conforming socket or cavity of a base member. A planar surface of the hemispherical or cylindrical is adjustable for relative tilt of a planar optical surface with respect to the base. A very small air bearing function facilitates relative sliding movement on the spherical interface between the components. A mechanical gimbal mechanism, offset from the nominal plane of the optical components by a four-point pushrod linkage arrangement, permits control of the tilt angle between the two optical components, affecting control of the beam deflection angle. This invention does not provide an easily controllable actuation force especially for small angles.
Swain et al's U.S. Pat. No. 4,961,627 illustrates a hemispherical element with a mating conforming socket or cavity of a base member. A planar surface of the hemispherical element is adjustable for relative tilt of a planar optical surface with respect to the base for refracting a beam passing through the hemispherical element. As in Donelan, a fluid filled gap separates the hemispherical element from the base. Piezoelectric actuators about the perimeter of the device provide for relative tilting of one component to the other. As in Donelan it is a drawback that a fluid seal is required and that the beam passes through the fluid.
In summary, there remains room in the art for a compact optical beam steering or deflecting device or design that provides for a high mirror rigidity and a high degree of mirror flatness or surface figure accuracy such as are currently obtainable by conventional optical forming techniques using conventional optical materials. There is also a need for a steering mirror capable of steering higher beam powers with improved heat dissipation and with the highest obtainable reflectivity to avoid beam absorption. There is a further need for a highly reflective mirror, which is coatable, with know high reflectivity coatings by conventional processes. Moreover, in optical switching applications there is a need for a mirror latching capability for maintaining a mirror in a fixed position for relatively long time periods without consuming electrical power. Moreover, it would be a benefit if each mirror in an optical switch was know to be unmoved after a power interruption